The present invention is related to the field of optical communications, and more particularly to waveguide design in photonic integrated circuits.
Photonic integrated circuits (PIC) provide an integrated technology platform increasingly used to form complex optical circuits. The PIC technology allows many optical devices, both active and passive, to be integrated on a single substrate. For example, PICs may comprise integrated lasers, integrated receivers, waveguides, detectors, semiconductor optical amplifiers (SOA), and other active and passive semiconductor optical devices. Such monolithic integration of active and passive devices in PICs provides an effective integrated technology platform for use in optical communications.
A particularly versatile PIC platform technology is the integrated twin waveguide (TG) structure in which active and passive waveguides are combined in a vertical directional coupler geometry using evanescent field coupling. As is known, the TG structure requires only a single epitaxial growth step to produce a structure on which active and passive devices are layered and fabricated. That is, TG provides a platform technology by which a variety of PICs, each with different layouts and components, can be fabricated from the same base wafer. All of the integrated components are defined by post-growth patterning, eliminating the need for epitaxial regrowth. Additionally, the active and passive components in a TG-based PIC can be separately optimized with post-growth processing steps used to determine the location and type of devices on the PIC.
The conventional TG structure, however, suffers from the disadvantage that waveguide coupling is strongly dependent on device length, due to interaction between optical modes. A common problem in prior-art TG structures is the relative inability to control the lasing threshold current and coupling to the passive waveguide as a consequence of the sensitivity to variations in the device structure itself. The sensitivity variations arise from the interaction between the even and the odd modes of propagation in the conventional TG structure. This interaction leads to constructive and destructive interference in the laser cavity, which affects the threshold current, modal gain, coupling efficiency and output coupling parameters of the device. It is noted that the threshold current represents the value above which the laser will lase, the modal gain is the gain achieved by traveling through the medium between the laser facets, and the coupling efficiency is the percentage of optical power transference between the active and passive regions in the optical device. In sum, the conventional TG structure suffers from unstable sensitivity in performance characteristics due to laser cavity length, even/odd mode interaction and variations in the layered structure.
A modified TG structure was disclosed in U.S. Pat. No. 5,859,866 to Forrest et al., which addressed some of the performance problems of the conventional TG structure by adding an absorption layer (or loss layer) between the upper and lower waveguides, thereby introducing additional loss to the even mode so that its interaction with the odd mode is attenuated. That patent, which includes common inventors with the invention described herein, is hereby incorporated by reference herein. The modified TG structure described in the ""866 patent is designed to have relatively equal confinement factors for both the even and odd modes in each waveguide layer by constructing active and passive waveguides of equal effective indices of refraction. The resulting confinement factors are relatively the same because the even and odd optical modes are split relatively equally in the active and passive waveguides. The absorption layer in the modified TG structure suppresses lasing on the even mode, thereby making the TG coupling efficiency independent of laser cavity length. The absorption layer substantially eliminates the propagation of the even mode, while having minimal effect on the odd mode. With the substantial elimination of even-mode propagation by the absorptive layer, modal interaction is largely eliminated, resulting in optical power transfer without affecting performance parameters such as the threshold current, modal gain, coupling efficiency and output coupling.
However, the modified TG structure of the ""866 patent is ineffective in a device with a traveling-wave optical amplifier (TWA), which is an important component in PICs designed for optical communication systems. In a TG device with an absorption layer operated as a TWA, the additional absorption in the single pass through the active region is insufficient to remove the even mode. It is desirable to have a common optical structure that can be effectively utilized for integrating both lasers and TWAs.
Therefore, there is a need in the art of optical communications to provide a relatively simple and cost-effective integration scheme for use with a traveling-wave optical amplifier (TWA).
There is a further need in the art to provide a twin waveguide (TG) structure that ensures stability in the laser and the traveling-wave optical amplifier (TWA).
There is a further need in the art to provide a TG structure that significantly reduces negative effects of modal interference without the concomitant coupling loss.
There is a further need in the art to provide a TG structure with the aforementioned advantages that can be monolithically fabricated on a single epitaxial structure.
The invention provides an asymmetric twin waveguide (ATG) structure that significantly reduces the negative effects of modal interference and which can be effectively used to implement both lasers and traveling-wave optical amplifiers (TWA). The ATG in the invention advantageously ensures stability in the laser and the TWA. In addition, the ATG provided in the invention can be monolithically fabricated on a single epitaxial structure without the necessity of epitaxial re-growth. Most importantly, the ATG, according to the present invention, is a versatile platform technology by which a variety of PICs, each with different layouts and components, can be fabricated from the same base wafer and modified with conventional semiconductor processing techniques to produce substantial modal gains and negligible coupling losses between PIC components.
In an embodiment of the ATG structure of the invention, the effective index of one of the passive waveguides in the ATG is varied from that of a symmetric twin waveguide such that one mode of the even and odd modes of propagation is primarily confined to the passive waveguide and the other to the active waveguide. As a result, the mode with the larger confinement factor in the active waveguide experiences higher gain and becomes dominant.
In an illustrative embodiment, monolithic integration of a 1.55 xcexcm wavelength InGaAsP/InP multiple quantum well (MQW) laser and a traveling-wave optical amplifier (TWA) is achieved using the ATG structure of the invention. The laser and the amplifier share the same strained InGaAsP MQW active layer grown by gas-source molecular beam epitaxy, while the underlying passive waveguide layer is used for on-chip optical interconnections between the active devices. In this particular embodiment, the passive waveguide has a higher effective index than the active waveguide, resulting in the even and odd modes becoming highly asymmetric. An appropriate combination of the thickness and index of refraction of the materials chosen for the waveguides results in modifying the effective index of refraction. The ATG structure uses the difference in modal gains to discriminate between the even and odd modes.
In a further embodiment, the active waveguide in a monolithically integrated device is laterally tapered by conventional semiconductor etching techniques. The tapered region of the active waveguide, at a junction of active and passive devices, helps to reduce coupling losses by resonant or adiabatic coupling of the optical energy between the passive waveguide and the active waveguide. As a result, the modal gain is significant compared to the symmetric TG structure and the coupling loss in the non-tapered ATG structure is reduced to negligible levels.